Biosensors provide an analysis of a biological fluid, such as whole blood, urine, or saliva. Typically, a biosensor analyzes a sample of the biological fluid to determine the concentration of one or more analytes, such as alcohol, glucose, uric acid, lactate, cholesterol, or bilirubin, in the biological fluid. The analysis is useful in the diagnosis and treatment of physiological abnormalities. For example, a diabetic individual may use a biosensor to determine the glucose level in whole blood for adjustments to diet and/or medication.
Biosensors may be implemented using bench-top, portable, and like measurement devices. The portable measurement devices may be hand-held. Biosensors may be designed to analyze one or more analytes and may use different volumes of biological fluids. Some biosensors may analyze a single drop of whole blood, such as from 0.25-15 microliters (μL) in volume. Examples of portable measurement devices include the Ascensia Breeze® and Elite® meters of Bayer Corporation; the Precision® biosensors available from Abbott in Abbott Park, Ill.; Accucheck® biosensors available from Roche in Indianapolis, Ind.; and OneTouch Ultra® biosensors available from Lifescan in Milpitas, Calif. Examples of bench-top measurement devices include the BAS 100B Analyzer available from BAS Instruments in West Lafayette, Ind.; the Electrochemical Workstation available from CH Instruments in Austin, Tex.; another Electrochemical Workstation available from Cypress Systems in Lawrence, Kans.; and the EG&G Electrochemical Instrument available from Princeton Research Instruments in Princeton, N.J.
Biosensors usually measure an electrical signal to determine the analyte concentration in a sample of the biological fluid. The analyte typically undergoes an oxidation/reduction or redox reaction when an input signal is applied to the sample. An enzyme or similar species may be added to the sample to enhance the redox reaction. The input signal usually is an electrical signal, such as a current or potential. The redox reaction generates an output signal in response to the input signal. The output signal usually is an electrical signal, such as a current or potential, which may be measured and correlated with the concentration of the analyte in the biological fluid.
Many biosensors include a measurement device and a sensor strip. The sensor strip may be adapted for use outside, inside, or partially inside a living organism. When used outside a living organism, a sample of the biological fluid is introduced into a sample reservoir in the sensor strip. The sensor strip may be placed in the measurement device before, after, or during the introduction of the sample for analysis. When inside or partially inside a living organism, the sensor strip may be continually immersed in the sample or the sample may be intermittently introduced to the strip. The sensor strip may include a reservoir that partially isolates a volume of the sample or be open to the sample. Similarly, the sample may continuously flow through the strip or be interrupted for analysis.
The measurement device usually has electrical contacts that connect with the electrical conductors of the sensor strip. The electrical conductors typically connect to working, counter, and/or other electrodes that extend into the sample reservoir. The measurement device applies the input signal through the electrical contacts to the electrical conductors of the sensor strip. The electrical conductors convey the input signal through the electrodes into the sample present in the sample reservoir. The redox reaction of the analyte generates an output signal in response to the input signal. The measurement device determines the analyte concentration in response to the output signal.
The sensor strip may include reagents that react with the analyte in the sample of biological fluid. The reagents may include an ionizing agent to facilitate the redox reaction of the analyte, as well as mediators or other substances that assist in transferring electrons between the analyte and the conductor. The ionizing agent may be an oxidoreductase, such as an analyte specific enzyme, which catalyzes the oxidation of glucose in a whole blood sample. The reagents may include a binder that holds the enzyme and mediator together.
Many biosensors use amperometric methods where an electrical signal of constant potential (voltage) is applied to the electrical conductors of the sensor strip while the measured output signal is a current. Thus, in an amperometric system current may be measured as a constant potential is applied across the working and counter electrodes of the sensor strip. The measured current then may be used to determine the presence of and/or quantify the analyte in the sample. Amperometry measures the rate at which the measurable species, and thus the analyte, is being oxidized or reduced at the working electrode. In addition to analytes, biological substrates and mediators, for example, may serve as measurable species
As the time during which the input signal is applied to the sensor strip increases, the rate at which the measurable species is oxidized or reduced at the working electrode decreases. Thus, after an initial period of high current output, the current recorded from the sensor strip decreases as the input signal continues to be applied. This current decrease with time may be referred to as an electrochemical decay, and the rate of this decay may be correlated with the concentration of measurable species, and thus the analyte, in the sample. An electrochemical decay may be a transient or Cottrell decay.
The electrochemical decay may be correlated with the analyte concentration in the sample by expressing the decay with an equation describing a line that relates current with time by the natural log function (In), for example. Thus, the output current may be expressed as a function of time with an exponential coefficient, where negative exponential coefficients indicate a decay process. After the initial decrease in current output, the rate of decrease may remain relatively constant or continue to fluctuate.
U.S. Pat. No. 5,942,102 (“the '102 patent”) describes the relationship between measured output current and time during a conventional analysis. An electrical signal is input to a sensor strip about 60 seconds after introduction of the whole blood sample to the strip. Initially, a rapidly decreasing current is observed, which is followed by a relatively constant or “steady-state” current output that is generated by the feedback of mediator from the counter to the working electrode. The feedback of the mediator provided by the short distance between the electrodes results in the current becoming substantially independent of time after the initial decrease. In this conventional analysis, the analyte concentration of the sample may be determined from the concentration and diffusion coefficient of the mediator as determined by: (1) measuring current as a function of time; and then (2) estimating the steady state current.
While the analysis method described in the '102 patent relies on the steady-state portion of the current decay, U.S. Pat. No. 6,153,069 (“the '069 patent”) and U.S. Pat. No. 6,413,411 (“the '411 patent”) describe methods where the concentration of a mediator, and thus the underlying analyte, is determined from the diffusion coefficient of the mediator. These systems are configured to provide a rate of current decay that is described by the Cottrell equation.
Current measurements demonstrate Cottrell decay when the measured current is inversely proportional to the square root of time. Current measurements with Cottrell decay may be described by the Cottrell equation given below as Equation (1):
                                          i            ⁡                          (              t              )                                =                                                                      nFAC                  b                                ⁡                                  (                                      D                                          π                      ⁢                                                                                          ⁢                      t                                                        )                                                            1                /                2                                      =                                                                                nFAC                    b                                    ⁡                                      (                                          D                      π                                        )                                                                    1                  /                  2                                            ⁢                              t                                  -                  0.5                                                                    ,                            (        1        )            where i is the measured current; Cb is the bulk concentration of the electrochemically active species in mol/cm3; A is the electrode area in cm2; F is the Faraday constant of 96,500 coul/equivalent; n is the number of electrons transferred in equivalents/mol.; D is the diffusion coefficient in cm2/sec; and t is the time of the electrochemical reaction in seconds. Thus, the Cottrell equation describes current as an exponential function of time, having a decay constant or exponential coefficient of −0.5. Further details of the Cottrell equation and the boundary conditions required for Cottrell behavior may be found in chapter 5, pp. 136-45, of Electrochemical Methods: Fundamentals and Applications by Bard and Faulkner (1980).
A system designed to operate with a Cottrell current decay requires a decay constant of −0.5. An electrochemical system demonstrating a −0.5 decay constant implies that the requirements of a Cottrell current are present, namely that the analyte has completely converted to a measurable species and that a substantially constant concentration distribution of this measurable species occupies the sample reservoir before current measurement. These requirements are further described in the '069 and '411 patents.
Column 4, Lines 39-40 of the '411 patent discloses that initial incubation periods of 15 to 90 seconds, preferably from 20 to 45 seconds, are used for glucose testing. After the initial incubation period and application of a single excitation input signal, current measurements demonstrating Cottrell decay may be recorded from 2 to 30 seconds or preferably from 10 to 20 seconds following application of the input signal to the sensor strip. The requirement of a longer initial incubation period also is depicted in FIG. 7 of the '411 patent, where the sample was allowed to react in the sensor strip (incubate) for 160 seconds before application of the input signal.
The longer incubation periods required to completely convert the analyte to measurable species provide: (1) time for hydration of the reagent layer containing the reagents; and (2) time for the reagents to convert the analyte. For example, column 4, lines 36-44 of the '411 patent describes an incubation period of sufficient length to allow the enzymatic reaction to reach completion. After this incubation period, where the glucose analyte is fully converted to a measurable species, the instrument imposes a known potential across the electrodes to measure the resulting diffusion limited (i.e. Cottrell) current at specific times during the resulting Cottrell current decay. Thus, the conversion of the analyte to the measurable species is completed before Cottrell decay is observed. Complete hydration of the reagent layer also is recognized in the '411 patent as a requirement for Cottrell decay. The '411 patent discloses that incomplete wetting of the reagent results in a failure of the system to follow the Cottrell curve decay, which results in an inaccurate analyte concentration value being obtained.
In addition to an extended incubation period, Cottrell decay also requires a substantially constant concentration distribution of a measurable species in the sample as the distance from the electrode surface increases. A substantially constant concentration distribution may be achieved with: (1) relatively large sample volumes; and/or (2) a relatively large distance between facing planar electrodes or substantially planar electrodes and the bottom surface of the sensor strip lid. For example, column 8, line 40 of the '069 patent describes a working electrode occupying a sample reservoir providing a 50 μL sample volume where the vertical distance between the working electrode and the lid is from 500-2000 μm. In another example, unlike the closely spaced electrodes of the '102 patent, the distance between the working and counter electrodes described in column 7, lines 62-66 of the '411 patent must be at least 100 microns, and preferably greater than 100 microns.
Conventional analysis methods typically lengthen the time required to analyze samples by requiring incubation periods, electrode distances, and sample reservoir volumes sufficient to allow the system to have Cottrell decay. Accordingly, there is an ongoing need for improved biosensors; especially those that more quickly determine the analyte concentration of a sample and do not rely on the estimation of a steady state current value. The systems, devices, and methods of the present invention overcome at least one of the disadvantages associated with conventional biosensors.